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From: Steven Rostedt <rostedt@goodmis.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: linux-kernel <linux-kernel@vger.kernel.org>,
	"Joel Fernandes, Google" <joel@joelfernandes.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	paulmck <paulmck@linux.ibm.com>,
	Boqun Feng <boqun.feng@gmail.com>,
	Will Deacon <will.deacon@arm.com>,
	David Howells <dhowells@redhat.com>,
	Alan Stern <stern@rowland.harvard.edu>,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates
Date: Fri, 16 Aug 2019 15:15:41 -0400	[thread overview]
Message-ID: <20190816151541.6864ff30@oasis.local.home> (raw)
In-Reply-To: <623129606.21592.1565975960497.JavaMail.zimbra@efficios.com>

On Fri, 16 Aug 2019 13:19:20 -0400 (EDT)
Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:

> ----- On Aug 16, 2019, at 12:25 PM, rostedt rostedt@goodmis.org wrote:
> 
> > On Fri, 16 Aug 2019 10:26:43 -0400 Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
> >   
> [...]
> >> 
> >> Also, write and read to/from those variables should be done with
> >> WRITE_ONCE() and READ_ONCE(), given that those are read within tracing
> >> probes without holding the sched_register_mutex.
> >>   
> > 
> > I understand the READ_ONCE() but is the WRITE_ONCE() truly necessary?
> > It's done while holding the mutex. It's not that critical of a path,
> > and makes the code look ugly.  
> 
> The update is done while holding the mutex, but the read-side does not
> hold that mutex, so it can observe the intermediate state caused by
> store-tearing or invented stores which can be generated by the compiler
> on the update-side.
> 
> Please refer to the following LWN article:
> 
> https://lwn.net/Articles/793253/
> 
> Sections:
> - "Store tearing"
> - "Invented stores"
> 
> Arguably, based on that article, store tearing is only observed in the
> wild for constants (which is not the case here), and invented stores
> seem to require specific code patterns. But I wonder why we would ever want to
> pair a fragile non-volatile store with a READ_ONCE() ? Considering the pain
> associated to reproduce and hunt down this kind of issue in the wild, I would
> be tempted to enforce that any READ_ONCE() operating on a variable would either
> need to be paired with WRITE_ONCE() or with atomic operations, so those can
> eventually be validated by static code checkers and code sanitizers.

My issue is that this is just a case to decide if we should cache a
comm or not. It's a helper, nothing more. There's no guarantee that
something will get cached.

-- Steve


> 
> If coding style is your only concern here, we may want to consider
> introducing new macros in compiler.h:
> 
> WRITE_ONCE_INC(v) /* v++ */
> WRITE_ONCE_DEC(v) /* v-- */
> WRITE_ONCE_ADD(v, count) /* v += count */
> WRITE_ONCE_SUB(v, count) /* v -= count */
> 

  reply	other threads:[~2019-08-16 19:15 UTC|newest]

Thread overview: 60+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-18 10:29 WARNING in tracepoint_probe_register_prio (3) syzbot
2019-08-16  0:11 ` syzbot
2019-08-16 14:26   ` [PATCH 1/1] Fix: trace sched switch start/stop racy updates Mathieu Desnoyers
2019-08-16 16:25     ` Steven Rostedt
2019-08-16 16:48       ` Valentin Schneider
2019-08-16 17:04         ` Steven Rostedt
2019-08-16 17:41           ` Mathieu Desnoyers
2019-08-16 19:18             ` Steven Rostedt
2019-08-16 19:19             ` Alan Stern
2019-08-16 20:44               ` Joel Fernandes
2019-08-16 20:49                 ` Thomas Gleixner
2019-08-16 20:57                   ` Joel Fernandes
2019-08-16 22:27                     ` Valentin Schneider
2019-08-16 22:57                       ` Linus Torvalds
2019-08-17  1:41                         ` Mathieu Desnoyers
2019-08-17  4:52                         ` Paul E. McKenney
2019-08-17  8:28                           ` Linus Torvalds
2019-08-17  8:44                             ` Linus Torvalds
2019-08-17 15:02                               ` Mathieu Desnoyers
2019-08-17 20:03                                 ` Valentin Schneider
2019-08-17 23:00                                   ` Paul E. McKenney
2019-08-19 10:34                                     ` Valentin Schneider
2019-08-17 22:28                             ` Paul E. McKenney
2019-08-20 14:01                           ` Peter Zijlstra
2019-08-20 20:31                             ` Paul E. McKenney
2019-08-20 20:39                               ` Peter Zijlstra
2019-08-20 20:52                                 ` Paul E. McKenney
2019-08-16 21:04                   ` Linus Torvalds
2019-08-17  1:36                     ` Mathieu Desnoyers
2019-08-17  2:13                       ` Steven Rostedt
2019-08-17 14:40                         ` Mathieu Desnoyers
2019-08-17 15:26                           ` Steven Rostedt
2019-08-17 15:55                             ` Mathieu Desnoyers
2019-08-17 16:40                               ` Steven Rostedt
2019-08-17 22:06                                 ` Paul E. McKenney
2019-08-17  8:08                       ` Linus Torvalds
2019-08-20 13:56                         ` Peter Zijlstra
2019-08-20 20:29                           ` Paul E. McKenney
2019-08-21 10:32                             ` Will Deacon
2019-08-21 13:23                               ` Paul E. McKenney
2019-08-21 13:32                                 ` Will Deacon
2019-08-21 13:56                                   ` Paul E. McKenney
2019-08-21 16:22                                     ` Will Deacon
2019-08-21 15:33                                 ` Peter Zijlstra
2019-08-21 15:48                                   ` Mathieu Desnoyers
2019-08-21 16:14                                     ` Paul E. McKenney
2019-08-21 19:03                                     ` Joel Fernandes
2019-09-09  6:21                           ` Herbert Xu
2019-08-16 20:49                 ` Steven Rostedt
2019-08-16 20:59                   ` Joel Fernandes
2019-08-17  1:25                   ` Mathieu Desnoyers
2019-08-18  9:15                   ` stable markup was " Pavel Machek
2019-08-16 17:19       ` Mathieu Desnoyers
2019-08-16 19:15         ` Steven Rostedt [this message]
2019-08-17 14:27           ` Mathieu Desnoyers
2019-08-17 15:42             ` Steven Rostedt
2019-08-17 15:53               ` Mathieu Desnoyers
2019-08-17 16:43                 ` Steven Rostedt
2019-08-16 12:32 ` WARNING in tracepoint_probe_register_prio (3) syzbot
2019-08-16 12:41   ` Sebastian Andrzej Siewior

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